In additive manufacturing or rapid prototyping, arbitrary shapes are created, usually by layering. In the layering process, cross sections are generated, and covered with a feedstock, which is bonded to the preceding layers. As this operation is performed repetitively, the object geometry is produced.
Many layered manufacturing processes such as stereolithography, laser engineered net shaping and selective laser sintering employ little or no pressure during bond formation. Instead, they employ liquid wetting and liquid-solid phase transformations as the means of producing joining between previously built materials and added layers. These methods have certain engineering drawbacks when attempting to build structures using high melting point materials such as metals. Solid state bonding methods exist for layered manufacturing, but typically involve the use of modest amounts of pressure. Examples include ultrasonic welding, resistance welding and friction joining. During bonding, this pressure may result in deformation in the material subjected to the process.
As each cross section of the object is fabricated, the total area or shape may change. As a result, unless the feedstock employed has an infinitely small dimensions in X, Y, and Z, the potential exists for mismatch between the edges of the desired part cross section and the coverage of the feedstock. In most rapid prototyping processes, this results in the well known “stair stepping” of the surface; i.e., the feedstock does not perfectly match the contours of the part geometry.
One method for eliminating this stair stepping is to trim the part following addition of feedstock material. Correctly implemented, this can result in smooth part surfaces that exactly match part design intent. It also expands the range of feedstock geometries which are reasonable, to include much wider tape, strip or sheet formats. The wider the feedstock, the higher productivity can be, as less process time is consumed by material feeding, traveling over the part cross section and bonding of layers.
However, as the feedstock dimensions increases from wire, to tape to sheet format, the amount of material to be trimmed increases as well. If deformation of the feedstock occurs during bonding of material increments to previously deposited layers, this may result in relatively large ratios of bonded, i.e., deformed material, to unbonded, i.e., undeformed material for certain cross section geometries. This situation is illustrated in FIG. 1. Outlines of feedstock material are shown at 102, and the outline of a cross-section to be covered is shown at 104. Zones 106 are where high ratios of undeformed material exists. Item 108 is a tape-type feedstock covering the cross-section.
The result of this situation is that a residual stress is produced in the deformed material which must be balanced by forces from the undeformed material. As a result there will be a tendency for movement of the tapes or sheet which are applied. Buckling of the material at the deformed-undeformed interface may also occur. This can cause non-uniform build through gaps between tapes, or deviations in surface height cause by the buckling of the material.